1. Field of the Invention
The present invention relates to a superlattice avalanche photodiode and more particularly, to a superlattice photodiode with the mesa structure which has a semiconductor superlattice carrier multiplication layer and semiconductor electric-field buffer layers disposed at each side of the carrier multiplication layer.
2. Description of the Prior Art
High-speed response, low dark current and high-reliability semiconductor photodetectors are essentially required for high-speed, high-sensitivity and high-reliability optical communications systems.
Recently, since silica optical fibers have the low-loss wavelength region of 1.3 to 1.6 .mu.m, InP/InGaAsP avalanche and pin photodiodes that enables to improve their operation speed and sensitivity within this wavelength region have been actively researched and developed.
The InP/InGaAsP avalanche photodiodes that have been put into practical use have the gain-bandwidth (GB) product of approximately 40 to 80 GHz and the maximum gain bandwidth of approximately 8 GHz.
However, with the conventional InP/InGaAsP avalanche photodiodes, the InP avalanche carrier multiplication layer has a small ionization rate ratio (.alpha./.beta.) of approximately 2, where .alpha. is an impact ionization rate of electrons and .beta. is an impact ionization rate of holes. Therefore, the conventional InP/InGaAsP avalanche photodiodes have a GB product limited to about 80 GHz or lower and a large excessive noise factor X of 0.7, which means that they have a limit of enhancement in operation speed and sensitivity without noise increase.
This limit applies to the other conventional photodetectors having multiplication layers made of a bulk III-V compound semiconductor.
As a result, to realize a low-noise avalanche photodiode with an increased GB product, it is necessary to artificially increase the ratio (.alpha./.beta.) of the hole and electron ionization rates or coefficients.
To realize the artificial increase of the ratio (.alpha./.beta.), a conventional improved avalanche photodiode was developed by F. Capassor et al., which was disclosed in Applied Physics Letters, Vol. 40, No. 1, pp 38-pp 40, 1982. In this photodiode, the energy discontinuity .DELTA.Ec of superlattice semiconductor layers in the conduction band is utilized to artificially increase the impact ionization increase of electrons.
Practically, an ionization rate ratio increase was confirmed for the GaAs/GaAlAs superlattice structure. Specifically, (.alpha./.beta.)=8 was obtained for the GaAs/GaAlAs superlattice structure, while (.alpha./.beta.)=2 for the bulk GaAs.
Also, another conventional improved avalanche photodiode was developed by Kagawa et al., which was disclosed in Journal of Quantum Electronics, Vol. 28, No. 6, pp 1419-pp 1423, 1992. This photodiode has an InGaAsP/InAlAs superlattice structure similar to that of Capasso et al.. The InGaAsP/InAlAs superlattice structure is sensitive to light with 1.3 to 1.6 .mu.m wavelength which has been employed in the long-distance optical communications. (.alpha./.beta.)=10 was obtained for the InGaAsP/InAlAs superlattice structure, while (.alpha./.beta.)=2 for the bulk InGaAs.
FIG. 1 shows a cross-section of the conventional improved avalanche photodiode developed by Kagawa et al.. In FIG. 1, the photodiode 600 has an n.sup.+ -InP buffer layer 602 formed on an n.sup.+ -InP substrate 601. An n.sup.- -InGaAsP/InAlAs superlattice carrier multiplication layer 603 is formed on the n.sup.+ -InP buffer layer 602. A p.sup.+ -InP electric-field buffer layer 604 is formed on the n.sup.- -InGaAsP/InAlAs superlattice carrier multiplication layer 603. A p.sup.- -InGaAs light-absorbing layer 605 is formed on the p.sup.+ -InP electric-field buffer layer 604. A p.sup.+ -InP cap layer 606 is formed on the p.sup.- -InGaAs light-absorbing layer 605. A p.sup.+ -InGaAs contact layer 607 is formed on the p.sup.+ -InP cap layer 606. These stacked semiconductor layers 602, 603, 604, 605, 606 and 607 constitute a mesa structure.
The surface of the mesa structure and the exposed surface of the substrate 601 are covered with a SiN passivation film 610. The passivation film 610 has a contact hole 610a on the top of the mesa structure. A polyimide film 611 is formed on the SiN passivation film 610 to bury a depression formed adjacent to the mesa structure.
A p-side electrode 609 is formed on the polyimide film 611 and the SiN passivation film 610 to be in Ohmic contacted with the p.sup.+ -InGaAs contact layer 607 through the contact hole 610a.
an n-side electrode 608 is formed on the back surface of the substrate 601 to be in Ohmic contacted therewith.
The semiconductor layers 602, 603, 604, 605, 606 and 607 constituting the mesa structure are epitaxially and successively grown by a gas-source molecular beam epitaxy (MBE).
With the conventional superlattice avalanche photodiode shown in FIG. 1, the conduction band edge discontinuity .DELTA.Ec is 0.39 eV and the valence band edge discontinuity .DELTA.Ev is 0.03 eV, which means that .DELTA.Ec is greater than .DELTA.Ev. Therefore, an obtainable energy for electrons due to the conduction band edge discontinuity is greater than that for holes due to the valence band edge discontinuity. Therefore, the electrons reach their ionization threshold energy more readily than for the holes, thereby increasing the ionization rate of the electrons. As a result, the ionization rate ratio (.alpha./.beta.) can be increased and therefore, the noise can be relatively decreased.
Further, still another conventional improved avalanche photodiode was developed by Nakamura et al., which was disclosed in ECOC, TuC5-4, pp 261-pp 264, 1991. This photodiode has a mesa structure containing an InGaAs/InAlAs superlattice structure similar to that of FIG. 1. The surface of the mesa structure is covered with a polyimide film to be contacted therewith.
The conventional avalanche photodiode shown in FIG. 1 has the following problem:
Interfacial states and crystal defects exist at the interfaces of the carrier multiplication layer 603, electric-field buffer layer 604 and light-absorbing layer 605 with the passivation film 610 and in their vicinity. The interfacial states and crystal defects tend to increase due to an applied bias voltage on a high-temperature reliability test and/or a normal operation with the increasing operation time, which leads to increase in leakage current. Accordingly, an existing dark current becomes from 0.8 .mu.A to several microamperes or greater to raise the noise level. PA1 Typically, there are interfacial states at the interfaces of the semiconductor layers 602, 603, 604, 605, 606 and 607 and the passivation film 610, the number of which is 2.times.10.sup.12 /cm.sup.2.eV. These interfacial states are grouped into some types. PA1 One of the types of the interfacial states is due to dangling bonds generated at the interfaces of the semiconductor layers 602, 603, 604, 605, 606 and 607 with the passivation film 610 during ordinary fabrication processes for growing the layers 602, 603, 604, 605, 606 and 607. PA1 Another type of the interfacial states is due to dangling bonds generated at the interfaces of the semiconductor layers 602, 603, 604, 605, 606 and 607 with native oxide films formed thereon after the formation process of the mesa structure. PA1 Still another type of the interfacial states is caused by the crystal defects existing on the surfaces of the semiconductor layers 602, 603, 604, 605, 606 and 607. PA1 The semiconductor layers 603, 604 and 605 are depleted by an applied reverse bias voltage. Since the p.sup.- -InGaAs light-absorbing layer 605 has the smallest band gap within the three layers 603, 604 and 605, it is considered that the major energy levels are caused by the dangling bonds existing at the interface of the light-absorbing layer 605 and the passivation film 610. PA1 Also, since the n.sup.- -InGaAsP/InAlAs superlattice multiplication layer 603 contains aluminum (Al) atoms having a tendency of producing a native oxide, it is considered that the major energy levels are caused by the dangling bonds existing at the interface of the multiplication layer 603 and the passivation film 610.
As a result, a problem that the raised noise level due to the dark current increase tend to cancel the noise reduction effect caused by the ionization rate ratio improvement occurs.
The above interfacial state increase is produced by the following reasons:
When the conventional photodiode of FIG. 1 is operated, a leakage dark current flows at the surface of the mesa structure (i.e., the interfaces of the semiconductor layers 602, 603, 604, 605, 606 and 607 with the passivation film 610) through the above interfacial states. Also, to induce the avalanche breakdown phenomenon in response to incident light irradiated along an arrow A through the passivation film 610, a high electric field is applied to the p.sup.- -InGaAs light-absorbing layer 605 due to a reverse bias voltage applied across the n- and p-side electrodes 608 and 609.
The typical strength of the applied electric field is approximately 500 to 600 kV/cm for the InGaAsP/InAlAs carrier multiplication layer 603, and approximately 100 to 200 kV/cm for the InGaAs light-absorbing layer 605.
The leakage dark current flowing at the surface of the mesa structure serves like hot carriers due to the high electric field, and is injected into the passivation film 610 to be stored therein. Due to this phenomenon, the surface of the mesa structure, i.e., the interfaces of the semiconductor layers 602, 603, 604, 605, 606 and 607 with the passivation film 610, deteriorate and at the same time, the surface potential of the mesa structure varies to thereby increase the leakage dark current.
The increase of the surface leakage dark current due to the hot carrier injection is prominent at the interface of the InGaAs light-absorbing layer 605 having a small energy band gap with the passivation film 610. This is a chief cause for the dark current increase with the time.
In addition, the increase rate of the leakage dark current varies strongly dependent upon the injection energy of the hot carriers, in other words, the electric-field strength in the InGaAs light-absorbing layer 605. For example, when the electric-field strength of the light-absorbing layer 605 is 100 kV/cm or less, an obtainable lifetime of the photodiode is 10.sup.5 to 10.sup.6 hours. When it is 100 to 150 kV/cm or greater, an obtainable lifetime is 10.sup.4 hours or less. Consequently, to increase the lifetime of the conventional photodiode of FIG. 1, it is necessary that the electric-field strength of the light-absorbing layer 605 is kept at 100 kV/cm or less.
However, if the thickness and/or doping concentration of the semiconductor layers 602, 603, 604, 605, 606 and 607 are set so that the electric-field strength of the light-absorbing layer 605 is kept at 100 kV/cm or less, the following new problem occurs.
Due to the electric-field strength reduction, the frequency response of the conventional photodiode of FIG. 1 deteriorates at the low bias voltage in which an obtainable carrier multiplication rate is low. As a result, the dynamic range for the high-speed response becomes extremely narrow.